“Some Branch Davidians may have died from inhalation of hydrogen cyanide, which is produced when CS burns under certain conditions, and others may have been asphyxiated or incapacitated by the CS itself.”

So they may have got hold of some materials looted from various places and there are elements there -- they might be industrial chemicals, for example -- that could be put together to make a chemical called hydrogen cyanide, which is a lethal chemical agent.